Chondrodite

Chondrodite is biaxial(+), with refractive indices variously reported as nα = 1.592 - 1.643, nβ = 1.602 - 1.655, nγ = 1.619 - 1.675, birefringence = 0.025 - 0.037, and 2V measured as 64° to 90°, calculated: 76° to 78°. Refractive indices tend to increase from norbergite to clinohumite in the humite group. They also increase with Fe and Ti and with (OH) substituting for F. Dispersion: r > v.

The chondrodite structure is based on a slightly distorted hexagonal close packed array of anions O, OH and F with metal ions in the octahedral sites resulting in zigzag chains of M(O,OH,F)6 octahedra. Chains are staggered so that none of the independent tetrahedral sites occupied by Si has OH or F corners. Half of the octahedral sites are filled by divalent cations, principally Mg, and one tenth of the tetrahedral sites are filled by Si. There are three distinct octahedra in the array: Fe is ordered in the M1 sites but not in the larger M2 and smaller M3 sites. Ti is ordered in the M3 positions, which are the smallest, but Ti concentration appears never to exceed 0.5 atoms Ti per formula unit in natural specimens. In the humite series Mg is replaced by Fe, Mn, Ca and Zn in that order of abundance, though Mg always predominates.